Claim of Unilateral Mistake Confers Right to Repossess Non-Financed Car?

A quick contracts/property/tort/consumer law hypothetical for incoming first-year law students (and their professors) to ponder over:

Car dealer sells a used pick-up truck to Buyer for $8,100 and a trade-in. Buyer pays the full amount by personal check and drives the truck off the lot. Dealer then calls Buyer at home and tells him that they looked up the wrong number in their book; the truck actually costs $10,000 more. Dealer tells Buyer that either he has to return the car and they’ll pay him $500 for his trouble, or he needs to cough up the extra $10,000. Buyer refuses.

In the middle of the night, the Dealer comes and “repossesses” the truck from Buyer’s driveway. Buyer’s trade-in is returned to him. Buyer’s check is not cashed.

Dealer claims that the contract was invalid because “one party ma[de] a mistake, and the other party knew or should have known that a mistake was made.” (See here for more details from WTVF-Nashville, and note the video link on the upper right. The file-dropping bit seems right off the Daily Show.) Assume that the Blue Book value of the truck is $21,240, and the trade-in was worth only a nominal amount.

Discuss; was there a valid contract? What claims does Buyer have, and even more important, what remedies should he get?